Temperature rising

From the lush Carolinian zone in the south to the spectacular boreal of the north, climate change threatens to bring forest ecosystems to the brink of collapse. Douglas Hunter reports on how global warming is changing Ontario’s landscape

by Douglas Hunter

In 1998, Stephen Murphy began noticing strangers in his neck of the woods. An associate professor in the department of Environment and Resource Studies at the University of Waterloo, whose expertise includes forest ecology and the struggle between invasive and native species, Murphy is intrigued to have found 11 tulip-tree saplings – normally limited to Ontario’s Carolinian zone – growing around Kitchener and along the west side of Waterloo. Also called yellow poplar Liriodendron tulipifera is a broad-leaf deciduous tree with unmistakable tulip-like flowers. Millions of years ago, when the climate was much warmer, tulip-trees could be found as far north as the Arctic. Today, its range is limited to parts of the eastern United States and the lower limits of southwestern Ontario.

It appeared that the tulip-trees Murphy came across were not deliberately planted.

Does their presence mean the entire Carolinian ecosystem is shifting northward? These trees could be “preliminary indications of movement where I am,” Murphy observes. “It’s sporadic, but it’s enough to make you think: I didn’t see that before.” Murphy suspects the reason for the tulip-tree’s migration is a change in the local “thermoperiod,” with longer stretches of warm weather year-round for several years in a row.

Murphy’s 11 tulip saplings are part of a broad basket of evidence suggesting that ecozones are changing in response to global warming. It is hard to say with confidence that any particular local ecological change can be attributed exclusively or even partially to global warming, especially given the cyclical nature of environmental elements such as temperature, water levels and species population dynamics. Ecosystems are complex, and the factors involved in the local successes of particular plant and animal species are numerous and intertwined in ways we do not fully understand.

Still, evidence of change driven by global warming is growing and is particularly persuasive in Canada’s north. The Arctic is warming at almost twice the rate as the rest of the world, and in the next 100 years, the permafrost line could retreat 300 kilometres north. Sea ice is undergoing a pronounced retreat. Summer ice breakup on western Hudson’s Bay now occurs two weeks earlier than it did 20 years ago, and killer whales have moved their hunting into the bay. Hudson Bay polar bears could be extirpated by 2050. Plant life in the far north has begun to change. Deciduous shrubs like dwarf birch and green alder are displacing ground cover such as lichen and moss. The spread of shrubby vegetation could affect threatened herds of caribou that feed on lichen. The red fox will probably expand its range by outcompeting the less adaptable Arctic fox.

In Ontario, academic studies have been considering the impact of global warming in a broad range of ecological areas. Warmer lake water, for example, is expected to be deleterious to cold-water species such as lake trout. But determining the causes of ongoing changes in species numbers and ranges in Ontario is difficult. Moreover, while concerted efforts are being made to build species databases for Ontario, generally not enough information is available to draw definitive conclusions as to why some plants and animals in Ontario are thriving and expanding, while others are in retreat, during a period of what is believed to be incremental global warming. “As far as I’m aware,” says Murphy, “we don’t have that developed a database. It has come together on an ad hoc basis, but is becoming more systemic. We have an emerging sense of what we have.”

Climate change is one factor among many to consider in explaining population shifts and species appearances and disappearances. “We’re seeing changes in ranges for a number of birds,” says Ted Cheskey, manager of the Canadian Nature Network at Nature Canada. But he offers the mourning dove as an example of a species that both invites and resists global warming as a possible explanation for its population changes. The bird has expanded its range considerably, becoming far more prevalent in southwestern Ontario over the past 20 years. And reports of the dove’s breeding activity in the boreal forest have been increasing despite the species’ low tolerance for cold. “Its very large fleshy feet are vulnerable to freezing,” says Cheskey. “I’ve seen them lose parts of their toes. I suspect that a limiting factor to their northern range is persistent cold temperature.” The presence of bird feeders and agricultural activity may contribute to the mourning dove’s expanding range. If the mourning dove is moving northward because the boreal zone is in retreat due to rising temperatures, one would expect to see a coincident retreat in the boreal forest.

But uncertainties over current impacts of global warming should not be taken as evidence that global warming is not in fact under way. Temperature-depth profiles produced from drill holes northwest of Lake Superior indicate that surface temperatures there began increasing 200 years ago. Most sobering are the latest efforts to model the province’s future ecosystems, and to determine their composition and diversity circa 2050. According to University of Toronto forestry professor Jay Malcolm, a mean two-degree Celsius rise in global temperature by the middle of this century could bring Ontario’s forest ecosystems to the brink of collapse. The landscape you know now will look very different by mid-century.

In November 2005, the WWF-World Wide Fund for Nature of Gland, Switzerland published “Implications of a 2 C Global Temperature Rise for Canada’s Natural Resources.” Malcolm, with then post-doctoral fellows Danijela Puric-Mladenovic and Hua Shi, provided a chapter of that report titled, “Projected tree distributions, tree migration rates, and forest types in Ontario under a 2 C global temperature rise” that delivers a stunning assessment of the imminent future of the province’s forest ecosystems.

Their study, says Malcolm, “projects huge changes. In almost every location in the province, all six climate models we used show a shift of some kind in the forest type. One model, for example, shows black spruce moving entirely out of the province. The potential for change is enormous. ”

Rising temperatures will initiate changes in ecozones that will force the migration of tree species outside their normal ranges, and will simultaneously introduce new tree species from the south as the Carolinian zone shifts northward. But this shifting of ecozones is predicted to happen with such rapidity, and across such great distances, as to outstrip the pace at which tree species can migrate. As tree composition falls into disarray, so, too, will the niche species that rely on them.

Alterations to forest composition – whereby wooded areas could give way entirely to meadows – precipitate a domino effect throughout an ecosystem. Even animals that are regarded as highly adaptable to shifts in forest location and composition – birds, for example – will be affected. Certain bird species rely on forests dominated by specific tree types and cannot exist without them. Cheskey notes that a number of species are dependent on the boreal forest: red-breasted nuthatch, golden crown kinglet, and several species of warblers. While some are adaptable, others are tree-specific in their habitats. “The baybreasted and blackpoll warblers, for example, feed mainly on spruce budworm.”

A very large swath of wildlife relies on particular forest types for their habitats. The general biological rule is that the viability of any creature weighing more than one kilogram is threatened when the forest type changes. “Animals larger than a kilogram have a greater dependence on their specific habitat,” Murphy notes. “Migratory birds are more adaptable, but even they need wintering grounds with their required food and habitat.”

This could mean that in Ontario, moose and caribou populations would, according to one study, decline significantly, while white-tailed deer were likely to become abundant across Ontario and Quebec.

A global ecological shift is already underway toward the poles, with plants and animals moving upward at a rate of about six metres per year. This languorous pace, however, won’t be fast enough as the planet heats up. “People forget,” says Murphy, “that if the climate zone changes, plants and animals have to be able to respond. And those zones will move one hell of a lot faster than all of the elements in an ecosystem can. As you go farther north into today’s boreal zone, for example, the soil isn’t right for the forests that are supposed to move up. Plants and animals will disappear because the soil conditions, the insects and fungi, the hydrology won’t be there for them.”

Like other life forms, trees species vary in genetic makeup and geographical distribution. Tree ranges can shift, but only incrementally, through successive generations. “You’ve got a fundamental problem,” Murphy adds. “Things have to shift so quickly that you’re going to lose a lot of trees. You’ll have what happens after an ice age. It will be a chaotic situation, where the winners and losers are not going to be clear.”

Jay Malcolm’s field research has taken him as far away as the Amazon and Africa, studying the impact of forest fragmentation on biodiversity. Exploring the biological processes of forests triggered a consideration of how forests would respond to climate change. In the past, public imagination fixed on tropical rainforests as determining the fate of the planet. The Amazon’s steamy expanses were the “lungs of the planet.” Now we are learning that the planet’s fate could hinge on what happens to the massive forests in the northern hemisphere.

Tree populations move by casting seeds onto welcoming ground in which the soil chemistry, microclimate and drainage are appropriate to the species. The ideal climate zone may shift into terrain – clay, granite or sand, acid or alkaline – that are not suited to a species. The landscape humans have crafted will also restrict opportunities for movement. Rather than being a broad frontier along which tree species may shift in range, much of southern Ontario is fragmented into rectilinear woodlots separated by farmers’ fields, highways, hydroelectric power transmission corridors and subdivisions. Species may be unable to spread from one stand or area of protected growth to another.

The piecemeal nature of southern Ontario’s landscape raises concerns about how animal populations will cope with shifts in ecosystems. “For those species that can migrate with their habitat,” says Wendy Francis, Ontario Nature’s director of conservation and science, “we have to make sure we’ve got north-south and altitudinal connectivity. The linear nature of river corridors and protected areas is critical.” Such connectivity is a fundamental concern in conservation circles, whether the issue is caribou in the north or marten in southern Ontario. Without these corridors, species can end up isolated in habitat “islands,” doomed to extirpation as climatezones shift and leave them behind.

Much of our understanding of how quickly tree species can shift in range is derived from researching tree movement during the retreat of the last great glaciation, some 10,000 years ago. Studies based on tree pollen extracted from soil core samples (particularly in lake beds) have led to a paradoxical conclusion.

“To try to get trees to move fast enough in a modelling context is problematic when we consider how they kept up with the retreat of the glaciers,” explains Malcolm. Based on pollen evidence, trees in the postglacial period shifted northward at a furious clip. Recent studies suggest that refuges of small founder populations endured farther north than was thought previously. “It may be that the tree populations moved less quickly than we thought, based on perceived rates from pollen,” Malcolm says. These refuge populations, in other words, helped move forward the overall population much more quickly than a single expanding frontier could manage.

Studies of postglacial tree population movements do not give us much hope that the trees growing in today’s fragmented natural environment will successfully keep pace with northbound climate zones. Malcolm says tree species in the past have been capable of shifting their ranges anywhere from “a couple hundred metres, up to about a thousand metres a year.” But the models predict that under climate change, the ranges will be shifting at a rate of 3,000 to 5,000 metres a year. “Those rates belong to the weediest of plants, not trees. Cheek grass, for example, can move up to 6,000 metres a year. But it gets helped along in ways that many trees can’t, by soil in car tires, for example.”

Ontario, which sprawls over several degrees of latitude, occupies some of the most difficult real estate on the planet when it comes to climate change. Northern latitudes are expected to experience the greatest environmental disruptions as the earth warms up. “We’re among the most vulnerable countries in the world,” says Malcolm.

Malcolm predicts a serious depletion of maple trees in the Algonquin Provincial Park region by 2050. “Maples and hardwoods in general are mainstays of the ecology and industry of that area,” he notes. “It was a surprise that most models showed such a decline there.” Another alarming expectation is the utter transformation of the southern boreal region. “All the models [indicate a] potential for massive change, showing a Great Lakes – St. Lawrence zone shifting all the way north to Kapuskasing and along Highway 11.” Sedges and grasses, rather than the woodlands of the adjoining southern climate zones, could replace Ontario’s boreal forest, observes Murphy. “If you go to a city, it’s a pretty good idea of what’s going to happen. Cities create their own weather, and there’s a diminution of natural features. This is going to happen on a much more widespread scale.”

If trees are unable to migrate in accordance with shifting ecozones, then what? Some species may adapt to a warmer climate through natural selection and perhaps some human intervention in terms of genetic engineering and planting. But humans are unlikely to replant even small corners of Ontario in time, never mind all of it, and natural selection requires generations of winnowing out the less adaptive individuals. But the climate is projected to change much more quickly than trees reproduce. Looking ahead to 2050, Malcolm observes that “it seems like a long time to people, but the reproductive cycles of trees aren’t much different than ours in terms of generations, maybe even slower.

“[When] trees are in the wrong climate, they’re stressed, and that makes them more susceptible to pests and disease,” says Malcolm. Pests and disease migrate much more quickly than trees do – their ranges expand in kilometres, rather than metres, per year. Stressed trees will be extremely vulnerable to being overrun by hordes of pests and diseases. Forests, trying to grow where they no longer belong, may simply collapse, along with dependent plant and animal species. Malcolm’s study concludes that, in addition to such massive reductions in biomass, decreases in “species richness, and consequent changes to ecological processes” could occur.

“It’s unlikely that a particular animal species will [become] extinct,” says Murphy, “but its population will drop. A lot of animals, like trees, will have a heck of a time adjusting. Some will do well but won’t be able to spread that quickly into new territories.”

Filling in the forest gaps, says Malcolm, will be a preponderance of “early-successional shade-intolerant species such as birch and aspen,” which are colloquially considered “weed trees” for the way they quickly colonize open spaces with poor soil nutrients, particularly ones created by clearcutting practices. Such species could do so well that they inhibit the natural development of later-stage, shade-intolerant species in the evolution of a forest, which in the boreal region generally are conifers such as white and black spruce and balsam fir. Forests that fail to evolve are unable to develop a species-rich biological neighbourhood.

Murphy’s research indicates that during transition stages brought about by climate change, invasive species tend to flourish, taking advantage of ecological disturbances. Native flora and fauna may become locally extinct. There may be “cascade” effects whereby species that are not individually vulnerable to climate change are nevertheless at risk because of their interactions or dependence on species that are vulnerable.

Moreover, the forests in the northern hemisphere (including living plant matter, soil and forest litter) are believed to contain more carbon than is currently in the atmosphere. Plants store carbon as they take in carbon dioxide (CO2) from the atmosphere and release oxygen. As stressed forests fail, some of the carbon that is locked up in living plant fibre is added to the soil as the wood decomposes. But carbon will also be released back into the atmosphere in the form of carbon dioxide, the leading greenhouse gas, as tinder-dry stands of dead trees and the forest litter succumb to fire – wildfires are expected to increase in frequency and severity across much of North America. Thus a loss in living tree biomass could mean a substantial release of CO2 – current CO2 concentrations in the atmosphere are at their highest levels in 650,000 years. In short, forest disruption in the northern hemisphere will have a major impact on atmospheric carbon levels, feeding the runaway heating that could come once the two-degree temperature increase threshold is reached.

In 2006, American scientists Paul Higgins and John Harte found that “ecosystem responses to climate change can include important climate feedbacks even if plant migration is extremely fast.” Even where plants are able to move north to a suitable climate zone in the scientists’ migration simulation, in the process of abandoning one area for another, the plants leave dead, carbon-rich matter behind. The resulting “carbon release from biomass and soil exceeds the current amount of carbon in the atmosphere and therefore constitutes a major positive feedback to climate warming.” In fact, this projected carbon release would be so high that emission targets for human sources currently thought necessary to avoid a runaway warming effect would need to be “much lower…than currently believed.” As Malcolm further notes, “We don’t know if trees can move north of the present treeline. But if they do, will their presence lead to increased global warming?”

Malcolm is realistic about the limitations of the computer climate models that drive conclusions of studies like his, and he expects deviations. “Favourable microclimates could allow some trees to persist in certain places. The devil is in the details.” Overall, however, he does not doubt that the province’s forests will be subjected to major dislocations. “We do know that climate changes in the past have caused massive changes in forests. And we’re looking over the next few decades at the equivalent of the change that occurred from the postglacial period all the way to the present.”

“I can’t say what Ontario is going to look like in a couple thousand years,” says Murphy. “In the short term, I think we’re going to have a much less diverse ecosystem in the province. Do we really want to be responsible for forcing that change?”

Freelance writer and author Douglas Hunter is a frequent contributor to ON Nature. His story “The ghost cat,” appeared in the Winter 2006/2007 issue.